Jonathon Hi,
You are something of a rarity on here, a lot of the guys end up giving up the VR4s and moving on to EVOs. Firstly be prepared for the board members to start chiming in and chastising you for calling the VR4 a DSM. All VR4s were built in Japan and imported whilst DSMs were USDM cars built in the USA. Now there are a few double standards going on here in my opinion because on the one hand guys regularly extoll the virtues of stuff built in the good ole US of A whilst going on to say the build quality of DSMs is crap and that the VR4s are put together much better. Not being an American and not really knowing that much about DSMs I can't really comment.
What I will say is that the 4G63 is a f***ing awesome engine, probably one of the best engines ever built and if looked after will give excellent reliability. The issue with DSMs (build quality doubts aside) seems on the whole to be the owners. And whilst I don't want to suggest you are anything like them, I would say that one of the things that really does lead to the horror stories regarding DSMs is when people buy a car and without doing the slightest bit of preventative maintenance or fixing existing issues, throw down a wad of cash on injectors, ECUs, turbos, exhausts, yada yada yada!
It's a 20 year old car. Expect it to have issues and if you find none expect the gremlins to start showing their ugly heads fairly soon. Unless you have very good assurances backed up with some proof of a recent timing belt change, do a timing belt job. Old timing belts, especially those that have perished, got brittle or are contaminated with oil tend to snap and when they do your valves and piston heads will become very closely acquainted following which your wallet or cheque book will become good friends with the local parts store.
Change all your belts, change your fluids, change your spark plugs (and at least check the wires) check the bearings and bushings, listen for grinding from the transmission especially 2nd gear. Check your ECU and smell it! If it smells like fish, your capacitors have leaked and your ECU needs attention. Be wary of a bad idle, your throttle body may require a rebuild and if the issue is traced to a bad Idle Speed Controller (ISC) be careful. If these short out they can take your ECU with them. Check your exhaust manifold for broken studs and check the entire intercooling system for boost leaks. Check the fuel pump and sock and the tank in general and swap out the fuel filter for a new one.
In other words make sure everything is working great stock before you even start pricing up bolt ons. Again this is just my opinion but one I know a lot of others will share with me.
If by some stroke of luck (so large that it would necessitate being referred to as a miracle /ubbthreads/images/graemlins/grin.gif) there is nothing wrong with your car then start looking to build things up in stages. There was a magazine years ago that did a run on building up a VR4 and they dynoed the car after each successive component was installed. I believe it was "Turbo and Import Cars" or "Turbo and Compact Cars" something like that, (I can't recall!) In any event, you will find in general that the stock USDM vehicle has one major built in restriction that you will want to remove in order to really make gains with bolt ons. The stock SMIC is an awful bottleneck. Ditch it for a JDM stock front mount kit or some aftermarket FMIC kit for at least 30hp straight off the bat. The stock 450cc injectors are a bit small compared to EVO stuff and for 400 hp you'll want to look at using something like EVO 560cc injectors or a little larger depending on whether you want to use meth injection with your 93. Most guys go with bigger cams but 400 hp 'might' possibly be within reach on stock cams. An exhaust will help especially as you will want a bigger turbo. 400 hp has been made on the EVO III 16G but most mortal people won't manage that on a 16G framed turbo to be honest so maybe look at something like an FP Green if you want to avoid a lot of fabrication that goes with running some more unorthodox setups.
You will need a clutch to hold 400 hp. Dozens of options on that one so I won't start there, do some searching on here there have been three or four threads on clutches recently. ECMLink is a great setup but again a lot of guys on here are having great success with EVO 8 ECUs, Ostrich Emulators and the good old Keydiver DSMChips so for your fairly modest goals you might be able to save some cash here.
It is worth noting finally that 400 hp is about the reliability threshold of the stock engine. That's why a lot of guys like the EVO III turbos. You can get 300-350 hp all day with them, good tuning has made more than that and you can rag on a car that has supporting mods all day and it will still be pretty reliable. So if you are looking for a reliable car that you can drive all day, I'd be tempted to lower your power threshold goal and go with something like that. I did a big build on one of my Galants and for the other daily driver VR4 I'm mostly stock and have just a small 16G in the works to go on later this year.
Anyway just some ideas for you here and I'm sure the supporting cast will throw in more opinions when the guys on your side of the world wake up!
Paul.